Composite materials allow significantly more opportunity to engineer material properties than do homogeneous materials. Composite thin films can be built by laminating stacks of films, depositing films that are mixtures or metal alloys, or incorporating pieces of one material in a continuum of a second material. The present inventors have identified a need for composite thin films comprised of particles embedded in a filler matrix, in which the choice of materials for each component—particles and filler—dictates unique properties of the composite film that are not provided by the particles or the filler alone.
Some composite materials are made by embedding reinforcing particles, fibers, or fabric in a liquid polymer matrix that is cured in and around a reinforcing material. Fiberglass and carbon fiber reinforced plastics are examples, but these are not typically made into thin films.
In another known method, a thin film matrix is first formed, typically on a substrate, and then implanted with particles by pressing or rolling the particles into the surface of the matrix. However, the depth and density of the implanted particles in such materials may be limited, particularly for particles that are soft relative to the matrix and for particles that are very small. Particles implanted in this manner may also be easily dislodged or washed away due to weak mechanical retention.
Still another known process wherein particles are embedded in a matrix of a second material involves a sol-gel reaction. In a conventional sol-gel process for forming a monolithic film, a precursor solution is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension that is coated onto a substrate. Particles in the colloidal suspension then condense in a new phase, a gel, in which a solid macromolecule is immersed in volatile solvent. The gel is dried to remove the solvent, resulting in a highly porous xerogel that is then densified by sintering and/or alternative heat treatment to form a monolithic glass or ceramic film. U.S. Pat. No. 5,076,980 of Nogues et al. describes a similar process for making sol-gel monoliths. Others have proposed mixing particles of a second material into the sol-gel matrix to form a composite material.
For example, U.S. Pat. No. 6,492,014 of Rolison et al. describes mixing guest particles in a mesoporous gel matrix formed by sol-gel reaction to create a composite material that is then dried. U.S. Pat. No. 6,749,945 of Knobbe et al. describes entrapping particles of alumina (Al2O3) or titania (TiO2) larger than 1 micron in a matrix of organically modified silicate (Ormosil) formed by sol-gel reaction to achieve a thin-film composite. Sol-gel composites, however, have certain disadvantages. Possible component materials are limited to those that can withstand processing at high-temperatures. Particle sizes may be limited to those that can be held in a colloidal suspension. Furthermore, composites made using the sol-gel process exhibit poor particle-to-filler bonding, high porosity, and they are relatively soft materials.
U.S. Pat. No. 6,999,669 of Summers et al. describes using atomic layer deposition (ALD) to manufacture photonic crystals comprised of a phosphor matrix and a plurality of defect regions. The matrix is formed by infiltrating interstices in a defect-laden synthetic opal with zinc sulfide (ZnS), gallium phosphate (GaP), alumina (Al2O3), titania (TiO2), or another material using ALD, then chemically removing the synthetic opal, leaving behind a honeycomb-like structure comprising only the matrix and the defect structures. See also, J. S. King et al., “Atomic Layer Deposition in Porous Structures: 3D Photonic Crystals,” J. Applied Surface Science v. 244, pp. 511-516 (2005); and Jeffrey S. King et al., “Conformally Back-filled, Non-close-packed Inverse-Opal Photonic Crystals,” Advanced Materials v. 18, pp. 1063-1067 (2006). Summers '669 indicates that ALD was chosen for its ability to produce high quality films with low porosity and good optical qualities.
The present inventors have identified a need for improved thin film composite materials.